Police expect autopsy results to determine whether a bullet that killed a 19-month girl was fired by police or her father, who authorities say used the toddler as a shield in a gunbattle that lasted nearly three hours.

Both the girl, Susie Lopez, and her father, Jose Raul Pena (search), were killed as Pena dueled with police on Sunday. One officer was wounded.

On Monday, Police Chief William Bratton (search) said his officers were having a difficult time coping with the girl's death.

"This has been extraordinarily traumatic for them, wondering if they possibly contributed to the death of a child or maybe caused the death with their own gunfire," Bratton told a news conference.

He said officers followed regulations allowing them to fire "when it reasonably appears necessary" to protect themselves or others from death or serious injury.

The girl's mother, Lorena Lopez, said she pleaded with officers to hold their fire.

"He had problems with depression, his business was not doing well," Lopez told KNBC-TV. "I told them that he needed help, he needs a psychologist, but please don't shoot."

Bratton said the fatal shooting was only the second time a hostage had been killed since the department formed its SWAT team in 1967. In that time, SWAT officers have responded to 3,800 calls involving hostages or people barricaded inside buildings.

It was the second time in nine months that Los Angeles officers have fired on an assailant holding a hostage. In November, police shot a man who held a pregnant woman by the neck outside the Mexican Consulate. Officers pulled the woman away unharmed and the man later died.

Sunday night's killings followed a three-hour standoff in which Pena, 34, exchanged gunfire with officers three separate times.